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All of this seems to have gone towards creating a usable space; the loft felt fine during the recent heatwave.
That's reassuring to know!
I'll probably end up in exactly the same situation with the loft roof as we're constrained by the same stupid planning rules (this is a great example of something Labour could change to make it easier to insulate homes, right).
If so and if we can still afford wood fibre elsewhere we'd have space to go thicker above the under eaves storage and in the vertical walls at the rear.
That Indinature is interesting, I think I came across it when they were just starting out but they didn't have any products you could actually buy then. I'd looked at hemp jute insulation from another supplier but the density wasn't as good which is important for sound proofing, which is important to me, and they didn't give out any dB figures while Indinature's look good. U-values not quite as good though - a big part of the appeal of SteicoFlex 036 is you can't really get better from eco insulation - but could be a decent option for intra-floor insulation where that's not quite as important.
Part of the appeal of wood based insulation for me is the less tangible stuff (smell/feel etc.) so I like the idea of using the SteicoFlex everywhere, but it will add up.
Your low-E triple glazing - that helps keep heat in but not out right? Or have you reversed it?
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Fox
I obsessed over the build-up of our loft for ages, being terrified of ending up with an unusable sweat lodge in the summer.
Considered various wood fibre products and Frametherm 32 mineral batts, before deciding to use PIR alongside other measures, for the following reasons:
Decrement delay: at the maximum practical insulation thickness of 180mm possible in our loft (without taking the piss with ridge height or reducing room volume too much), using wood fibre would only have gained ~1hr of decrement delay, whilst achieving a significantly worse U-value (~0.18 for wood fibre vs ~0.11 for PIR). I decided that the 8.5 hours of calculated phase shift delay would be sufficient. I also think that too long a delay is a negative, as this will negate the effectiveness of night-purge cooling.
Cost: wood fibre is still several times more expensive than PIR in the UK, and we simply could not afford to spend an extra couple of £K regardless. I’ve however since discovered a new UK manufacturer of hemp-based insulation called ‘Indinature’; their prices are competitive with high-performance mineral wool like Frametherm 32 etc, with all the benefits of wood fibre. Too late for our build.
Mitigations: I used solar-reflective top coat on the flat sections of the loft roof, and raw cement slates (much lighter in colour than usual) on the pitched elements. Low-E triple glazing in all windows and roof lights, external blinds on the Veluxes. Opening windows positioned to allow for cross-ventilation.
All of this seems to have gone towards creating a usable space; the loft felt fine during the recent heatwave.
There is however a lot to be said against the environmental cost of using shit-tons of petrochemical-based insulation products instead of carbon-sequestering bio-based materials… I did use wood fibre to insulate the small section of solid brick parapet wall in the loft however, so remember this token nod when we’re all fucked in a few years’ time.